1. Field of the Invention
The present general inventive concept relates to an audio power amplifier usable with an audio power amplifier system, and more particularly, to an audio power amplifier that provides a virtual ground to reduce power on/off noise in an audio power amplifier system, and a method of processing a signal in the audio power amplifier.
2. Description of the Related Art
Generally, an audio power amplifier is used in a digital audio system. Class A, class B, class AB and class D are used as an audio power amplifier. Among them, the class D amplifier can reduce drops in amplification efficiency which occur in class A, class B, and class AB amplifiers. The class D amplifier uses a method of converting an audio signal into a pulse width modulation (PWM) signal using on and off switching of a switching circuit. Accordingly, the class D amplifier does not have data conversion loss, and a nonlinear component occurring in the analog part, from which the switching circuit is started, is overcome. Generally, an audio system using such a power amplifier is composed of a microcomputer controlling the entire system, a digital signal processor processing a digital signal, a PWM unit generating a PWM signal by comparing an audio signal and a carrier signal, and a power stage unit amplifying the voltage and current of the PWM signal using a power switch. The power stage unit is composed of a switching field-effect transistor (FET) and an output filter.
FIG. 1A is a circuit diagram illustrating a conventional switching FET.
In the switching FET, an n-type metal-oxide-semiconductor (NMOS) transistor QN and a p-type metal-oxide-semiconductor (PMOS) transistor QP are alternately and repeatedly turned on/off so as to output a pulse waveform having a large output as illustrated in FIG. 1B.
The pulse waveform illustrated in FIG. 1B passes through a carrier frequency removing LC filter which is a combination of an inductor L5 and a capacitor C62, as illustrated in FIG. 2, so as to be converted into an audio signal. The waveform converted at this time has a direct current (DC) value of VDD/2.
While the converted audio signal passes through a DC removing capacitor C60, a DC value is removed. Accordingly, the audio signal passes through an electromagnetic interface (EMI) removing filter 210 so as to be provided to a speaker or a headset.
The DC removing capacitor C60 is used in a conventional power stage output filter illustrated in FIG. 2 for removing DC. However, the capacity of the DC removing capacitor C60 may be increased in order to achieve audio band properties (low frequency properties).
The DC removing capacitor C60 for increasing the low frequency properties may generate a noise component such as a popping sound.
In conventional audio power amplifier arts, since a common ground is connected in a long line connecting devices such as an earphone and a speaker, EMI properties may deteriorate. In addition, since grounds having different properties are connected, a hum may occur.